We have identified a deficiency in the exercise induced increase in bone mass in stressed bones of premenopausal amenorrheic women. The lack of bone strengthening which usually occurs in this setting appears to lead to an increase in fracture rate particularly in women with delayed menarche. This problem is of considerable importance to an exercising female population. We wish to examine the effects of estrogen replacement in this group to determine if this normal physiologic response can be reinitiated with hormonal therapy. Sixty exercising amenorrheic ballet dancers will be allocated randomly into two treatment groups: (A) estrogen treated, (B) placebo. After two years treatment, the participants will continue in the study for one year. The women will be randomized into four subgroups: (A) hormone treated unchanged, (B) placebo treated unchanged, (C) hormone treated to placebo, and (D) placebo to hormone treated. A control group of 25 normal dancers will be followed. All subjects will receive 10OOg calcium/day during both studies. The response of the skeleton in exercising women will thus be examined on estrogen replacement to determine if a normal increase in bone mass occurs with therapy and determine of the pattern persists after withdrawal of therapy. Injury rates in all groups will be followed to determine if there is a relation to injury and compared to normal controls on no medication. Further understanding of the role of estrogen deficiency on the skeleton of the premenopausal exercising women would radically alter attitudes towards the importance of therapy to prevent injury and would lead to the formulation of effective treatment to prevent stress fractures, particularly in a young population.